The present invention relates to an hydraulic control system for an excavating device. More particularly, the present invention concerns an hydraulic control system which automatically proportions the available flow of hydraulic fluid between a plurality of fluid actuators when the fluid flow rate demand exceeds the available fluid supply.
There now exist numerous types of hydraulically operated construction machinery. For example, hydraulically powered cranes, excavating devices, rollers and the like are commonplace in the construction industry. Typically, these hydraulically actuated machines use hydraulic actuators or fluid power motors to perform a plurality of functions, which may occur simultaneously. As an example, in an excavator, the boom may be raised or lowered, the boom may be articulated, and an earth engaging bucket may be articulated relative to the distal end of the boom. Each of these functions is performed by a different hydraulic cylinder and all may occur simultaneously.
In the design of such hydraulically actuated machines, it is impractical to provide a driving engine and a fluid pressure source having sufficient flow capacity to accommodate all of the hydraulic actuators at their maximum demand flow rates. This impracticality is due in part to the expense and the weight of the engine and pumps which would otherwise be required. Accordingly, it is desirable to provide a fluid pressure source and associated driving engine having an output capacity which is less than the combined maximum flow required when each fluid actuator operates at maximum demand. However, when the total actuator demand exceeds the available pump output capacity, operation of the hydraulically actuated devices becomes erratic for the condition where total actuator demand exceeds the pump capacity: for example, the most highly loaded fluid motor may practically stop further extensional movement until a less highly loaded actuator has attained the end of its actuating stroke. Such abrupt changes in flow distribution between the plurality of actuators, cause an abrupt change in the behavioral characteristics of the machine as seen from the operator's station. Moreover, in close quarter maneuvers, such abrupt changes may lead to accidental damage of either the machine or an adjacent structure.
In the past, efforts have been made to match the output of a variable displacement pump to the flow demand requirements of a plurality of series connected, pressure compensated valves. In one example, the exhaust flow from the pressure compensated valves passes through a restriction which generates a differential pressure that operates an hydraulically controlled pilot valve. The pilot valve causes a corresponding movement of a fluid actuator that increases or decreases the displacement of a variable displacement pump. See, for example, U.S. Pat. No. 3,863,448, issued to Purdy on Feb. 4, 1975.
Such a device does not operate to adjust the actuation of the plurality of actuators when their combined flow exceeds the maximum available output from the variable displacement pump. Moreover, there is no maintenance of flow proportions associated with the plurality of fluid actuators.
Many of the problems discussed above are also present when a plurality of actuator control valves are connected in parallel flow relationship. In one prior device, a plurality of closed center control valves regulate a pneumatic pilot pressure in response to two parameters: the total flow rate demand of the fluid actuators and the maximum pressure acting on any one of the fluid actuators. See, for example, U.S. Pat. No. 3,987,622, issued to Johnson on Oct. 26, 1976.
Devices which sense the maximum load pressure exerted in an hydraulic system must be large and heavy in order to withstand very high hydraulic pressures which approach the maximum pressure for which a device is designed. Moreover, the components required to effect the control functions are also unduly expensive by virtue of the necessity of handling the high pressures and the requisite sealing problems.
In view of the foregoing, it will be apparent that the need continues to exist for a truly effective control system to regulate a plurality of simultaneously actuated hydraulic motors so as to proportionally adjust and slow the movement of all actuators while efficiently using pumps and providing a low pressure control system having an open center such that the pressurized fluid is continuously available without delay.